Hi,
I am currently choosing A levels to take that can help me study Law at university; I am thinking about oxbridge but not a necessity. So far I'm certain I want to do Maths, French and History but I don't really know what 4th a level to take or whether I need one at all. I am thinking about English Lit as I enjoy essay based subjects but don't know if the course would be too hard (my school does pre-u).
Any advice would be much appreciated

Maths, French and History is my exact combination right now and I love it. They are three very different disciplines so it gives you a nice break between subjects while giving you a breadth of knowledge. I haven't been looking into law specifically but I have heard from lecturers that lawyers fluent in other languages will be sought out especially after brexit. I've also heard some firms prefer lawyers who do a conversion course after studying a different subject as they have knowledge outside law so keep that in mind if you find yourself really liking history or french.

In regards to your 4th subject, if you have the option I wouldn't do one. Unis will only look at 3 so it will give you more time to focus on those subjects. If you don't have a choice it really doesn't matter what you choose so do whatever you enjoy. All a levels are the same amount of work and level of difficulty no matter what people may say.